diamond

An allotrope (see allotropy), or structural
form, of carbon. Diamond is the hardest substance
known, with a Mohs hardness of 10, and the least compressible. It is a better
conductor of heat at room temperature than any other material, and when
completely pure is transparent. These extreme properties make diamond technologically
very useful. Its hardness makes it useful as a cutting tool in industry
and surgery, and because of its excellent heat conductivity it is used as a heat sink to cool electronic components rapidly. These applications
are a direct result of the internal structure of diamond, coupled with the
fact that the carbon atoms in diamond are more closely packed than the atoms
in any other material.

The atoms in a crystal of diamond are bonded into one giant molecule. Each
carbon atom is covalently bonded to four
other carbon atoms, and so each carbon atom has four nearest neighbors.
That is, its coordination number is 4. When just four bonding pairs of electrons
surround an atom, they are arranged tetrahedrally. This is the case with
diamond. All outer electrons of the carbon atoms are involved in the formation
of covalent bonds. There is no possibility of delocalized or mobile electrons.
As a consequence, diamond cannot conduct electricity.

Diamond burns when heated in air to 900°C; in an inert atmosphere it
reverts to graphite slowly at 1000°C,
rapidly at 1700°C.

Natural diamonds occur in some ancient volcanic "pipes," such as those in
South Africa, Tanzania, and in the United States at Murfreesboro, Arkansas,
and have been recovered from the ocean floor off the Cape of Good Hope.
Microscopic diamonds have also been found in some meteorites.

Diamond stars

The interior of some white dwarf stars
is believed to consist of crystalline carbon – in effect, these would
be diamonds as big as small planets. In 2004, astronomers announced that
measurements they had made of BPM 37093, a white dwarf lying some 50 light-years
from the Earth in the constellation Centaurus, probably has a diamond core
about 4,000 km in diameter. BMP 37093 pulsates regularly. By studying these
pulsations, the researchers were able to reveal the hidden interior of the
white dwarf, just as seismograph measurements of earthquakes allow geologists
to learn about the interior of the Earth.

Diamonds in
meteorites

Diamonds have been found inside meteorites.
Some of these diamonds were formed by shock pressure during violent impacts
between asteroids in the asteroid belt. Shock-formed diamonds tend to be
very cracked and measure one or two millimeters in size.

Another type of diamond in meteorites occurs as tiny crystals measuring
only a few nanometers across. They contain an isotope of xenon that is very rare on Earth but common in supernova
remnants, suggesting that this is where they formed. Nanodiamonds are
also surprisingly abundant, making up about 3 per cent of the carbon in
meteorites in which they occur. If the same is true of carbon in interstellar
space, just a gram of the dust and gas in an interstellar cloud could contain
as many as 10,000 trillion nanodiamonds.

Black diamonds

So-called black diamonds, or carbonado, may be also have come from space.
Black diamonds are only found in Brazil and the Central African Republic
and, unlike other diamonds, are made of millions of diamond crystals stuck
together. They are porous, too, which is puzzling because it would have
been difficult for gas to become trapped in rocks at the depths at which
terrestrial diamonds were formed – about 200 km below the surface.
In a paper published in 2007, Stephen Haggerty, a geologist at Florida International
University, proposes that the black diamonds arrived from space in a kilometer-sized
rock between 2.6 billion and 3.8 billion years ago. At that time, South
America and Africa were one land mass, which could account for the diamonds
showing up on two continents today.